The present invention generally relates to the field of diagnostic spectroscopy, and more specifically, to a method and apparatus for providing a laser reference for Fourier transform spectrometers. More specifically, the present invention is a subsystem including a vertical cavity surface emitting laser (VCSEL) and selected components making the subsystem useable as a reference laser.
In interferometry such as that used for optical spectroscopy, reference lasers are used to provide the ability to obtain digitized interferogram points that are equivalently spaced in position, which is a requirement of Fourier transform algorithms. The industry standard reference is the helium neon laser because of its inherent lasing wavenumber stability and its relatively small size and low cost when compared to other gas lasers.
International Publication WO 00/49690 to Singh et al. and entitled xe2x80x9cCompact Wavelength-Independent Wavelength-Locker for Absolute Wavelength Stability of a Laser Diodexe2x80x9d discusses the wavelength stabilization of a laser diode by tapping a fraction of the laser diode""s output and passing it through a narrow band power splitter to two detectors. The signal from the two detectors is compared, and predetermined control signals are used to maintain a constant lasing wavelength. The need for wavelength stability for wavelength division multiplexed (WDM) transmission systems is disclosed. Application of laser diodes for spectroscopic purposes is not disclosed. In addition, the patent application disclosed wavelength control through the utilization of optical feedback that is obtained from a dedicated diode laser control system. The control of wavelength stability requires significant additional electronics.
International Publication WO 01/20371 to Watterson et al. and entitled xe2x80x9cWavelength Reference Devicexe2x80x9d describes an apparatus for use in calibrating a tunable Fabry-Perot filter or tunable VCSEL to a precise absolute frequency and maintenance of that frequency using optical feedback derived from a Michelson interferometer. The drawbacks of this method are that the optical feedback system must be included in any application of a VCSEL, and a method of absolute lasing wavelength determination must be available. The desirable low cost feature of VCSELs relative to existing laser reference technology is negated by the described optical feedback system. The VCSEL lasing wavenumber control is provided by an additional optical feedback system. Further, the apparatus of Watterson et al. relies on absolute frequency to maintain a lasing wavenumber.
U.S. Pat. No. 6,069,905 to Davis et al. and entitled xe2x80x9cVertical Cavity Surface Emitting Laser Having Intensity Controlxe2x80x9d describes the incorporation of a photo detector into a VCSEL package for the purposes of intensity control. This method focuses solely on optical power and intensity regulation and control, which is not critical for the purposes of the application of VCSELs as references for interferometric spectrometers. Davis et al. do not disclose the control and correction of VCSEL lasing wavenumber shifts.
U.S. Pat. No. 5,267,152 to Yang et al. and entitled xe2x80x9cNon-invasive Method and Apparatus For Measuring Blood Glucose Concentrationxe2x80x9d describes the use of solid state lasers as sources of electromagnetic radiation for the non-invasive measurement of blood glucose concentration. Yang et al. do not describe the use of a solid state laser as a wavenumber reference for interferometry. The control of the solid state laser current, voltage and temperature are discussed because the measurement of blood glucose concentration, as described in this patent, is dependent on these parameters.
U.S. Pat. No. 5,933,792 to Andersen et al. and entitled xe2x80x9cMethod of Standardizing a Spectrometerxe2x80x9d describes the use of a standardization sample to determine a characteristic shape, which embodies the difference in response of an instrument over time or between instruments, for absorbance and wavenumber calibration. The limitation of this method is that the characteristic shape is used to correct spectra obtained at later times or on different instruments. The spectra themselves are not inherently correct. The disclosed apparatus does not deal with wavenumber calibration through control and correction of the optical component that determines the spectral wavenumber axis. Instead, it requires a characteristic shape that embodies the spectral differences to correct the spectral wavenumber axis.
In addition, U.S. Pat. No. 5,933,792 discloses a method of standardizing a Fourier transform infrared (FTIR) spectrometer that uses a HeNe laser as its reference. It does not discuss the use of a VCSEL as the reference for the FTIR spectrometer. The HeNe laser has relatively high cost, high power, generates more heat and occupies a large volume relative to a VCSEL. The present invention discloses the method and apparatus of a subsystem or subassembly necessary for successful use of a VCSEL as a reference for an interferometer in optical spectroscopy.
The present invention is directed to a subsystem for use in interferometry for optical spectroscopy applications which makes possible the use of a vertical cavity surface emitting laser (VCSEL) to serve as an accurate and precise reference laser as an alternative to the industry standard HeNe laser. The present invention offers substantial cost, size, heat and power consumption reductions compared to the HeNe laser. In preferred embodiments, the present invention makes feasible the use of the VCSEL as a reference for an interferometer by incorporating electronics to drive the VCSEL, a photodetector sensitive to the VCSEL output, and an algorithmic wavenumber shift estimation and correction algorithm or method which utilizes a known sample.
A preferred embodiment of the present invention is a subassembly for use in an optical spectroscopy system. The subassembly preferably includes an interferometer having optical components for receiving light and passing light along a defined light path. The optical components preferably include a beamsplitter that separates the light from a source into two portions and means for introducing a pathlength difference between the two portions. A vertical cavity surface emitting laser, including electronics to drive the vertical cavity surface emitting laser and project a beam therefrom is preferably operatively mounted on the interferometer with the beam following the defined light path to act as a reference laser for the interferometer. The interference pattern of the laser is received by a photodetector so that pathlength differences and an accurate digitized interferogram may be constructed for a sample under analysis. The vertical cavity surface emitting laser preferably includes means for temperature control and means for current control connected thereto along with computing means having therein an algorithm for correcting wavenumber drift by the vertical cavity surface emitting laser.
In preferred embodiments, the algorithm for correcting wavenumber drift by the vertical cavity surface emitting laser includes factors derived from spectroscopic analysis of a reference sample utilizing the interferometer and vertical cavity surface emitting laser of the subassembly. At least a portion of the generated spectrum is then compared to a known spectrum for the reference sample. The comparison can include analysis of the relative difference between the generated spectrum and the known spectrum of the reference at selected wavenumbers. Other types of algorithms can be utilized which rely on such methods as employing a derivative-based determination of wavenumber location of spectral features, a center of gravity based determination of wavenumber location of spectral features, an interpolation-based determination of wavenumber of location of spectral features or a wavenumber shift versus wavenumber regression to determine shift correction.
In an alternative embodiment, the algorithm for correcting wavenumber drift by a vertical cavity surface emitting laser can be derived from multiple spectroscopic analysis of a reference sample having a known spectrum utilizing a second interferometer and a second vertical cavity surface emitting laser of the same type as that utilized in the subassembly. The algorithm derived from the similar system can then be utilized as a predictor for performance of the vertical cavity surface emitting laser in the subassembly.
The vertical cavity surface emitting laser of the present invention also preferably includes means for temperature control and means for current control connected thereto. The means for temperature control preferably includes a temperature measurement device to provide a feedback signal to the control circuit, a set point signal, a Wheatstone bridge to compare the feedback signal to the set point signal, proportional integral and derivative (PID) filter to provide the control properties of the circuit, a reference voltage supply, and a MOSFET to regulate the output of the circuit using the signal obtained from the PID filter. A temperature monitor can also be included in the circuit. The means for current control preferably includes a precision voltage power supply, a precision resistor with low temperature coefficient to convert the output of the precision voltage supply to current, and a current monitor.
The reference sample having a known spectrum is preferably selected to include at least one rare-earth oxide. The rare-earth oxide may be doped into a diffusely reflective substrate or alternatively doped into a transmissive substrate. Preferred earth oxides include erbium oxide, dysprosium oxide, holmium oxide or samarium oxide. In alternative embodiments, the reference sample having a known spectrum can be a rare gas emission lamp which is selected from a neon emission lamp, a krypton emission lamp, an argon emission lamp or a xenon emission lamp. The reference sample could also include one or more etalons that may be measured simultaneously or in series.
In preferred embodiments, the subassembly is mounted in a spectrometer system that includes a sample holder. The reference sample is preferably measured while positioned in the sample holder.